Transmission mechanisms for rolling vehicles under unmanned operation have been under development since the early automatic guided vehicles (AGV). Mechanism morphology varies, depending on the class of wheels, which can be conventional or omnidirectional. A conventional wheel is one with a simple disk geometry; omnidirectional wheels are of various types, notably Mekanum wheels, as disclosed in German Patent 822,660, Class 63d, Group 1, of Jul. 8, 1949 to Fuchs, consisting of a drum on whose periphery a plurality of idle rollers are located, with axes skewed with respect to the axis of the drum. An alternative class of omnidirectional wheels comprises ball wheels, as disclosed in U.S. Pat. No. 5,186,270 of Feb. 16, 1993 to West; U.S. Pat. No. 5,374,879 of Dec. 20, 1994 to Pin et al.; and U.S. Pat. No. 5,927,423 of Jul. 27, 1999 to Wada et al. The contents of the above documents are hereby incorporated by reference in their entirety.
Transmissions for conventional wheels can be of two kinds, depending on whether the axis of the wheel is fixed or articulated to the vehicle chassis. Fixed-axis wheels are restricted to vehicles with a mobility of two. This is identical to the mobility of manned terrestrial vehicles. Articulated wheels offer the possibility of a mobility of three, or full mobility, which thus allows for the driving of a platform arbitrarily on a flat floor, with two independent translations, in the x- and y-directions, plus a rotation in the plane, commonly referred to as the θ-motion.
Articulated wheels can be either idle or actuated. Articulated idle wheels have an offset between the steering and the driving axes. Articulated actuated wheels make use of two motors for their actuation, one for the steering and one for the driving, but can be either centered or offset, depending on whether the steering and the driving axes intersect or not. Offset wheels are termed caster (or castor) wheels.
Caster wheels are common in applications not requiring an actuated wheel, such as in trolleys, the offset providing for a turning moment that allows the suitable reorientation of the wheel axis, as needed for pure rolling. Applications requiring the actuation of the wheel permit the use of centered wheels, provided that the wheel axis is steerable by means of a motor.
Caster wheels are commonly used in the case of steerable wheels. For example, U.S. Pat. No. 5,576,947, dated Nov. 19, 1996 and issued to Wienkop, describes a wheel of the offset type, further developed in WO9812498 Patent, of Mar. 26, 1998 to Legrand and Slater. A modification of this layout is disclosed in U.S. published patent application 20010008985, of Jul. 19, 2001 to Wada. The contents of the above documents are herein incorporated by reference in their entirety. The aforementioned documents describe an actuation unit with one single wheel, which is both steerable and driveable by means of two identical motors of vertical axes. The steering and driving motions are transmitted to the wheel with the aid of one mechanism involving bevel gears with straight teeth, intended for the driving of the wheel about a horizontal axis. The wheel of U.S. Pat. No. 5,576,947 and PCT application WO9812498 has its axis of rolling offset with respect to the vertical axis of steering, while the wheel of U.S. published patent application 20010008985 bears one additional offset, that of the wheel plane, from the same steering axis.
Deficiencies of transmissions of the type described above are to be noted: a) the unnecessary single or double offset contributes to the complexity of the control of the overall vehicle; b) one of the two identical motors is dedicated to the steering, the other to the driving, thereby performing two quite disparate functions that require disparate control strategies, i.e., position control vs. velocity control; c) the offset prevents the reorientation of the wheel while the vehicle (rolling robot) is stationary, thereby transmitting to the vehicle a parasitic motion upon reorientation; and d) bevel gear trains with straight teeth are noisy, while their counterparts with spiral teeth, capable of a smoother operation, are more expensive, which makes them less common in the art.
An alternative layout of a transmission includes a two-wheel unit, with the two wheels mounted coaxially, but turning at independent rates. A system of this type is described in Carnegie-Mellon University's Pluto, developed by Hans Moravec, as reported in the Proceedings of the 1983 ASME Conference on Computers in Engineering, Las Vegas. Pluto is a rolling robot with three actuation units, each supplied with two identical motors to both steer the common axis of the wheels and drive one of the two wheels, the second wheel being idle. This transmission is different from that of U.S. Pat. No. 5,576,947, PCT Application WO9812498 and U.S. published patent application 20010008985, in that the motors with their speed reducers do not drive their assigned axes directly, but via a differential mechanism, which is made of bevel gears.
Deficiencies of the above described alternate transmission are to be noted: a) the two identical motors are intended for two disparate functions, steering and driving; b) bevel gears entail the shortcomings mentioned above; c) the driving of one single wheel limits the load-carrying capacity of the unit to one-half that provided by the two motors; and d) the driving of one single wheel implies uneven loading, which leads to uneven wear of the motors.
In yet another alternative layout of a transmission a two-wheel unit has each of its two identical motors driving each of the two wheels with its own transmission. This type of transmission is described in a technical paper in the Journal of Robotics and Mechatronics, Vol. 7, No. 3, 1995. A transmission of this type is intended to convert a rotation about the vertical axis of the motor into a rotation about the horizontal axis of symmetry of the wheels. This conversion is implemented by means of a worm-gear transmission, which works under friction, and hence, impacts on the efficiency of the device.
Consequently, there is a need in the industry for providing a transmission that alleviates, at least in part, the deficiencies associated with prior-art dual-drive transmissions.